Since inkjet image forming methods are simple and can produce images at low costs, the inkjet image forming methods have been used for various print industries. Among the inkjet image forming methods, methods using an ultraviolet crosslinking inkjet ink have advantages such that the ink is a quick drying ink (i.e., the ink can be dried in a short time); images can be formed on a recording material having little ink absorbability; and it is not necessary for a factory manufacturing an ultraviolet crosslinking inkjet ink to provide a facility for performing a toxic gas treatment so as to satisfy the toxic gas emission regulations. Further, since the content of a volatile organic component included in such an ultraviolet crosslinking inkjet ink is very low, the ink is environmentally friendly. Therefore, the inkjet image forming methods using an ultraviolet crosslinking ink have been broadly used.
Ultraviolet crosslinking inkjet inks for use in the inkjet image forming methods are broadly classified into radically polymerizable inks and cationically polymerizable inks. The radically polymerizable inks have an advantage over the cationically polymerizable inks such that the manufacturing costs are relatively low. Specifically, cationically polymerizable inks typically include a photo-acid-generating agent as a polymerization initiator to generate a strong acid upon receipt of light. However, since such photo-acid-generating agents have a complex chemical formula, the photo-acid-generating agents are not broadly used, and are therefore very expensive. Therefore, cationically polymerizable inks have relatively high manufacturing costs compared to radically polymerizable inks. In addition, photo-acid-generating agents generate a small amount of strong acid with elapse of time even when being preserved in a dark place. Therefore, ink flow passages of image forming apparatus using such a cationically polymerizable ink have to have good resistance to acids, and therefore the material used for the ink flow passages is limited.
In contrast, since photo-radical-generating agents (hereinafter referred to as photoradical generators) for use in the radically polymerizable inks have been broadly used, the photoradical generators have relatively low costs. In addition, since photoradical generators do not generate a strong acid, the radically polymerizable inks have good stability even when being used or preserved over a long time, and there is no limitation on the material of the ink flow passages.
Conventional radically polymerizable ultraviolet crosslinking inks typically use, as a radically polymerizable monomer, an acrylate, which has such advantages as to have a relatively low viscosity and to be easily crosslinked with a small amount of light.
However, acrylates typically have a high skin irritation potential and a high skin sensitization potential.
For these reasons, the inventors recognized that there is a need for a radically polymerizable ultraviolet crosslinking inkjet ink which has a high level of safety without using an acrylate and which can be easily crosslinked with a small amount of light.